Remember that age-old piece of nutritional advice that salty foods make you thirsty? It's wrong.
Scientists have long believed that eating more salt results in urinating more, which causes one to drink more to refuel. But no long-term study has ever looked into the matter to verify the seemingly logical assumption. Now an international team of scientists found evidence suggesting the contrary: Those who ate more salt retained more water, were less thirsty, and were hungrier.
The new study, published in two papers in The Journal of Clinical Investigation, looked at salt intake and water in the context of traveling to Mars. Astronauts need to have enough water and knowing how salt intake impacts water retention can help make more accurate calculations.
The researchers — from the German Aerospace Center (DLR), the Max Delbrück Center for Molecular Medicine (MDC), and Vanderbilt University — had two groups of male subjects locked into a mock spaceship. One group was there for 105 days and the other for over 205 days. Both groups had identical diets except for three varying levels of salt intake.
The scientists found evidence of the obvious: Eating more salt meant there was a higher content of salt in urine and more volume of urine. Salt is made up of sodium and chloride ions; the previous school of thought was that these ions would latch onto water molecules and travel through the body until they are discarded in urine. This explains the greater volume of urine after eating salty foods. With more water leaving your body, the feeling of thirst served as a biological reminder to drink more water.
This study also found that our bodies triggered a water retention process: Salt was delivered to the urine and water traveled back to the kidney and body. This means salty foods don't make you urinate water and, since you're not depleting water, you're not feeling thirsty — the study also found higher salt intake didn't result in greater thirst. In 2015, research conducted at the University of Haifa made similar preliminary findings: There was no correlation between salt intake and increased sense of thirst.
In the new study, higher salt consumption did make the subjects complain more about being hungry, however. Overall, the study reversed the way scientists looked at salt's effect on the body and debunked an old adage accepted by the average consumer.
"[Urea] not solely a waste product, as has been assumed," Prof. Friedrich C. Luft, MD of the Charité and MDC says in a statement. "Instead, it turns out to be a very important osmolyte — a compound that binds to water and helps transport it. Its function is to keep water in when our bodies get rid of salt. Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt."
So the bag of chips you're binging on won't make you guzzle sugary drinks, but it may make you want a cookie.